We will use two model systems to study DNA replication in human cell lines. 1. The role of the Epstein-Barr virus (EBV) nuclear antigen I (EBNA-1) in the formation of a replication barrier at the EBV family of repeated sequences in human cell lines will be studied using an in vitro replication system. We will determine the minimum number of repeats that are required to produce a barrier to replication in vitro and determine whether EBNA-1 binding to the family of repeats inhibits helicase activity of the T antigen. We will then determine whether cellular proteins can also produce a replication barrier by binding to the EBV family of repeats. 2. We will identify replication origins in the rDNA gene cluster that replicate early or late during the S phase. We will determine the location of the sites at which the replication of the immunoglobulin heavy chain multigene family initiates. This will be determined in cell lines in which one or more of these genes are expressed and in cell lines in which they are transcriptionally silent. We will use deletion analysis to determine the sequences that are critical for these immunoglobulin replication origins to function. Targeted integration mediated by homologous recombination will be used to modify the chromosomal origin region.